1. [Field of the Invention]
The present invention relates to a multilayer ceramic substrate containing interconnected circuit patterns which is used in an electronic industry, such as in the manufacture of electronic devices or articles for domestic use or computers, etc.
More specifically, the present invention relates to a low-temperature fired multilayer ceramic substrate with interlayered silver conductors and copper external conductors.
2. [Description of the Prior Art]
Heretofore, circuit substrates using ceramics as an insulator have been used in various electronic devices or equipments.
As typical examples, there have been known co-fired type multilayer ceramic substrates using tungusten (W) or molybdenum (Mo) as conductors for wiring in which the substrates have been co-fired with the W or Mo conductors in a reducing atmosphere in order to prevent oxidation of these conductors. However, such conventional substrates are disadvantageous in the following aspects due to the use of the W or Mo conductors.
(1) High conductor resistivity of 10 to 20 milliohms/square. PA0 (2) Au plating is needed for obtain solderability. PA0 (3) Conductors are subjected to oxidation when highly reliable resistors of RuO.sub.2 type or Bi.sub.2 Ru.sub.2 O.sub.7 type which need to be fired in an oxidizing atmosphere are formed.
In recent years, there have been developed cofireable multilayer ceramic substrates using Ag base conductors, such as Ag, Ag-Pd, Ag-Pt, or Ag-Pd-Pt, which conductors have a low conductor resistivity and are firable in an oxidizing atmosphere together with a ceramic material as an insulator which can be fired at temperatures below the melting points of the conductive materials.
Copper base conductors have been also known and the conductors are most suitable as external conductors because of their advantages of low conductor resistivity of the order of 1.5 to 3 milliohms/square, good anti-migration property and high solder-leach resistance.
When an Ag base conductive material is used as the conductors in the aforesaid multilayer ceramic substrate, a metal component, such as Ag, etc., in the conductors formed on the external surface of the multilayer substrate tends to migrate and thereby serious problems, such as decrease of insulation resistance or short circuit, may be caused. Further, the Ag conductors do not have a sufficient solder-leach resistance. As a measure of preventing such a migration of Ag, etc.,for example, in Ag-Pd system conductors, palladium content is increased. However, in such a case, the conductor resistivity will be increased to an unfavorable levels, for example, the orders of 20 to 30 milliohms/square and difficulties have been experienced in obtaining Ag external conductors having well-balanced properties in all the aspects of the properties of conductor resistivity, migration resistance and solder-leach resistance, from Ag conductive materials.
On the other hand, when Cu is used as conductors in the multilayer ceramic substrate, firing is carried out at relatively low temperature of the order of 800.degree. to 1100.degree. C. and a neutral atmosphere or a reducing atmosphere is employed in order to prevent the oxidation of copper. In such firing conditions, an organic binder employed to make ceramic insulator materials or other powdered materials castable can not be fully burned off and residues of the resin binder component remain as carbon, thereby causing a serious decrease of insulation resistance. Therefore, a very prolonged binder removal procedure is necessary and this is undesirable for practical use. RuO.sub.2 type or Bi.sub.2 Ru.sub.2 O.sub.7 type resistors which have been known as highly reliable resistors can not be employed with the Cu conductors. The reason for this is that these resistors should be fired in an oxidizing atmosphere, while such an atmosphere causes oxidation of Cu.